The current casting methods for fabricating dental prostheses contain many steps where errors or inaccuracies can occur and accumulate. In general, an impression of the teeth surface and the adjacent areas is taken with an impression material that is typically a polymeric elastomer. This impression is then used for making a cast stone or plaster die. A wax model of a prosthesis is built on this die. The model is removed and invested in refractory material. Once the refractor has set, the wax is removed by heating to make the refractory mold. The metal is then cast into the mold. The accuracy of this process is greatly influenced by the materials used, the laboratory techniques, and the skill of the technicians. Errors incorporated in each stage can accumulate to cause misfit of the prosthesis. The strength, durability, and biocompatibility of the prosthesis are clearly determined by the available casting metals.
Electroforming as an alternative to casting in dentistry has been described by a number of investigators. Electroforming has the immediate advantage of being able to replicate surface features of less than 1 .mu.m which, in general, will not be affected by subsequent processing. More than three decades ago, Rogers and Armstrong reported on a combined electroforming-casting method. Rogers et al., J. Prost. Dent., 11: 959-960 (1961). They electroformed gold on a silver-painted die stone. After electroforming the matrix, gold was cast onto the matrix to form an inlay. Rogers later reported that the gold matrix was removed by heating the matrix and the die, followed by rapid cooling to disinteg rate the die. Rogers, Austr. Dent., 15: 316-323 (1970); U.S. Pat. No. 3,997,637 (Rogers). In subsequent years, Rogers reported on the gold electroforming casting technique. Rogers, Austr. Dent. J., 21: 479-487 (1976); Rogers, Austr. Dent. J., 22: 100-106 (1977); Rogers, Austr. Dent. J., 4: 163-170 (1979); Rogers, Austr. Dent. J., 25: 1-6 (1980); Rogers, Austr. Dent. J., 25: 205-208 (1980). Tettammati et al. reported on crowns prepared by electrodeposition. Tettammati et al., Rev. Circ. Argent. Odontol., 32-35 (1969).
In 1971, Wismann was granted U.S. Pat. No. 3,567,592 for an electroplated cermet alloy. In this patented process, a die stone coated with lacquer is metallized in a copper or silver solution. The die is then electroplated in a solution containing ceramic and metallic particles. A metal particularly well suited for this process is nickel. Later, Wismann in 1984 was granted U.S. Pat. No. 4,488,590 on electroforming of gold on dies made of a low-melting-temperature alloys that subsequently could be easily removed by moderate heating. The die was first electroplated with a thin layer of copper or nickel. After completion of the electroforming process, the die was removed by melting, and the nickel or copper substrate was chemically removed in an electrolyte that would not attack the gold.
Many reports were published on electroforming of dental prostheses: de Freitas, Rev. Bras. Odontol., 30: 96- 102 (1973); French Patent No. 2,316,356 (Serfaty, 1977); Sifaoui, Rev. Fr. Prothese Dent., 62-68 (1980); J. P. Kokai 8355592 A2 (Mitsara, 1983); Vrijhoef et al., Restorative Dentistry, 1: 143-146 (1985); Renggli et al., Dental-Labor (Munch), 33: 1655-1657 (1985); and DE 38 09 435 A1 (Yamashita et al.). Hayashi, Kanagawa Shigaku, 20: 58-75 (1985), and Chung, Kanagawa Shigaku, 22: 32-49, report a metal plate denture electroformed of Ni-CO alloy. In Watanabe et al., Kanagawa Shigaku, 23: 622-628 are reported. Kober et al., Zahnarztl Mitt., 77: 2406-2409. Klett et al. (1987), Dtsch. Zahnarztl Z., 42: 614-617; Klaus, Quintessenz Zahntech, 14: 1229-1240 (1988); Menne, Quintessenz Zahntech, 15: 1057-1064 (1989); and Stroppe, Dental-Labor (Munch), 38: 201-202 (1990), report work concerning electroforming of metals for dental prostheses. However, all of the alloys studied--gold, nickel, or nickel-cobalt--are either not strong enough to be used in thin sections where substantial strength is needed, or they lack biocompatibility.